d3_selection_enterPrototype.select = function(selector) {

  var subgroups = [],

      subgroup,

      subnode,

      upgroup,

      group,

      node;

  for (var j = -1, m = this.length; ++j < m;) {

    upgroup = (group = this[j]).update;

    subgroups.push(subgroup = []);

    subgroup.parentNode = group.parentNode;

    for (var i = -1, n = group.length; ++i < n;) {

      if (node = group[i]) {

        subgroup.push(upgroup[i] = subnode = selector.call(group.parentNode, node.__data__, i));

        subnode.__data__ = node.__data__;

      } else {

        subgroup.push(null);

      }

    }

  }

  return d3_selection(subgroups);

};